The resonant patterns of radio-frequency (RF) passive functions, often consisting of assemblies of coils and capacitors for frequencies of less than 1 GHz, may be glued, screwed or soldered to the printed circuit. This latter situation is the most frequent; an inductor will be connected by soldering it to the printed circuit, for example.
The patterns may also be etched, most often by chemical etching, into the conductive layers of the printed circuit. This is the technique the most widely adopted for producing microwave resonant circuits, for frequencies of greater than 1 GHz. Thus it is known to implement by chemical etching passive functions (filters, couplers, resonators) on the printed circuit (microstrip technology) or in the printed circuit (stripline technology).
The standard technologies used for “tuneable” filter functions for frequencies of less than 1 GHz are most often based on the use of inductors of helical shape necessitating the use of high-precision mechanical components leading to high costs and despite all this a tolerance on the value obtained that often necessitates an adjustment phase after manufacture, either by adaptation of the inductor itself or by changing values of surrounding components such as the capacitors.
As a general rule, RF power equipment is of reducing cost and overall size, for ever increasing requirements in respect of reliability. Also, this RF equipment is increasingly constituted of a stack of cards, and therefore of somewhat plane mechanical structures.
There therefore exists at present a requirement for printed circuits implementing different functions, including filter functions, the printed circuits complying with criteria of small thickness, high performance and low induced costs as well as of technically easy manufacture.